Musnad Ahmad 1026:1

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah, Nafi' narrated to me, from Safiyyah, from some of the wives of the Prophet, may Allah bless him and grant him peace, from the Prophet, may Allah bless him and grant him peace, who said: "Whoever goes to a fortune-teller and believes him in what he says, his prayer will not be accepted for forty days."